In electronic information devices, by using optical transmission to send and receive large amounts of information such as image data, efforts are being made to achieve high-speed processing while avoiding electromagnetic noise. A key elemental technology in such optical transmission are optical waveguide-based optical interconnections within the device for enabling signal transmission by converting electrical signals from an LSI chip or an integrated circuit, for example, into light. In internal optical interconnections, a 30 to 100 μm multimode waveguide is optimal to facilitate positioning between the optical input/output of the optical waveguide and the light-receiving/light-emitting elements. Polymer waveguides, which have a low optical loss, are easy to mass produce, and can be easily integrated with electrical circuit boards, are attracting attention as such multimode waveguides.
However, in recent years, there has been a demand for ever smaller sizes in information devices and terminals such as cell phones and mobile devices. To address this demand and achieve smaller device sizes, flexible printed circuits are being used by bending the circuit carrier and placing it in the narrow space within a small housing, and device configurations in which display and main unit can be folded together using hinges are being adopted.
When interior optical interconnection technology utilizing a polymer waveguide is employed in such downsized information devices and terminals, the polymer waveguide that is formed on a flexible printed circuit (hereinafter, also referred to as “FPC”) is required to have flex resistance so that it does not fail when repeatedly folded at a small radius of curvature. Also, when a photoelectric conversion element or the like is surface mounted on the FPC in which a polymer waveguide has been formed, the polymer waveguide must have a heat resistance capable of withstanding the high-temperature reflow conditions for lead-free solder. In addition, the polymer waveguide is required, of course, to have a good transparency with low optical loss.
The acrylic resins that are widely used in the manufacture of optical fibers are known as polymer materials for forming such polymer waveguides. However, because acrylic resins have a low heat resistance, they are unable to withstand the high-temperature reflow conditions for lead-free solder. Therefore, in cases where acrylic resin was used to form an optical waveguide on a substrate and an attempt was made to mount a photoelectric conversion element or the like at a light-receiving or light-emitting portion of the optical waveguide, it was found to be impossible to utilize a mounting process that employs a reflow operation. For this reason, it was necessary to position the waveguide core by aligning it, on an order of several tens of micrometers, with a photoelectric conversion element or the like that had been mounted beforehand on another substrate. Such a mounting process is very cumbersome and hardly conducive to mass production.
Methods of forming optical waveguides using curable resins such as epoxy compounds, which are polymeric material having a high heat resistance and also having a good interlayer adhesion, are also known.
For example, Patent Document 1 below discloses art for fabricating an optical waveguide, wherein a liquid photocurable resin solution containing an oxetane resin, an epoxy resin and a photopolymerization initiator is coated onto a substrate surface, following which a cladding or core is formed by carrying out pattern exposure and development.
Patent Document 2 below discloses art for forming an optical waveguide core, wherein a liquid photocurable epoxy resin containing a polyfunctional reactive oligomer and a photopolymerization initiator is used as a polymeric material to form a cladding or core, and this photocurable epoxy compound solution is coated onto a silicon substrate, following which pattern exposure and development are carried out.
Patent Document 3 below discloses a resin film for forming optical materials such as optical waveguides, which film is obtained by applying a varnish composed of a phenoxy resin or a solid epoxy resin dissolved in a solvent onto a substrate, then removing the solvent.
However, because the glass transition temperature (Tg) of epoxy resins is too high, such resins have basically a poor flexibility. Even when polymer materials such as those disclosed in Patent Documents 1 to 3 are used, they lack sufficient flex to withstand use as an optical waveguide in a hinge.